Fixed hang when called with no args or STDIN piped data

[luizberti]

Will check if STDIN is associated with an interactive fd using isatty(), from either io.h when on Windows or unistd.h when on POSIX. If no data is being piped through STDIN, and no arguments were passed, will print usage and return 0; just like calling it with -h would.

[luizberti]

This addressed issues brought up by #177 and #178

/cc @saper @xzyfer

[xzyfer]

Code formatting aside I'm ok with this. If specific implementation issues arise we can deal with them on a case by case basis, but IMHO this is an ideal approach from the user's perspective.

[xzyfer]

@am11 any thoughts on the Windows side of things?

[am11]

nit: print_usage and return statements in their own lines.

[am11]

One nit, otherwise LGTM 👍

[luizberti]

@am11 Sorry about the formatting, too used to having quick checks in a single line. Will fix.

@xzyfer As for Windows concerns: It was already including io.h on Windows builds, which is what contains isatty and fileno but with leading underscores. Hence those 2 defines.
As for POSIX, I just added an else to the same WIN32 ifdef that includes unistd.h, which is where POSIX keeps isatty and fileno.

It's not a big change and it should work as expected, Microsoft documentation did not mention any behaviour differences from the POSIX version, and the io.h was made exactly for providing an IO interface that is compliant with POSIX but works on Windows.

[xzyfer]

Thanks heaps @LuizFB! Thanks again @am11.